Electronic level-sensitive apparatus



Nov. 25, 1952 H. F. TAPP x-:r AL 2,619,620

ELECTRONIC LEVEL-SENSITIVE APPARATUS Filed Nov. 28, 1950 5 Sheets-Sheet l @gz WQ-2- Nov. 25, 1952 H. F. TAPP Er AL 2,519,620

f ELECTRONIC LEVEL-SENSITIVE APPARATUS Filed Nov. 28, 195o v 5 shets-shee/t 2 271 00a/: JHEs/l BY A ToRN s Nov. 25, 1952 H. F. TAPP El' AL 2,619,620

ELECTRDNIC LEVEL-SENSITIVE APPARATUS A Filed Nov. 28, 1950 5 Sheets-Sheet I5 Tf/fooo/Pr LI Hfs/f ATTORNE S Nov. 25, 1952 H. F. TAPP E" AL ELECTRONIC LEVEL-SENSITIVE APPARATUS 5 Sheets-Sheet 4 Filed Nov. 28, 195o INVENTORJ HMRYE TAPP mvo Tf1/foam: J. Hfs/f Nv. 25, 1952 H. F. TAPP Er AL 2,619,620

ELECTRONIC LEVEL-SENSITIVE APPARATUS Filed NOV. 28, 1950 5 Sheets-Sheet 5 INVENToRs HARRY E111/P Ma T//faoaf JI/Yfs/f BY n# ATTORN Ys Patented Nov. 275, 1952Y ELECTRNIC LEVEL- SENSITIVE APPARATUS Harry F. Tapp, Longmeadow, and Theodore J.

Mesh, Easthampton, Mass., assignors to Gilbert & Barker'Manufactui-ing Company, West lSpringlela;l,.Mass., a. corporationof Massachusetts Application November 2S, 195i), Serial No. 197,964

(Cl. S18-31) 13 Claims.

This invention relates to an improved electronic level-sensitive apparatus for use with containers of fiuent material either for the purpose of indicating the level of the material in the container or Vfor the purpose of maintaining the material at a predetermined level in the container.

The invention has for an object the provision in an apparatus of the class described of a levelsensitive element,-which is spaced vabove the surface of the fluent -material in the container and is automatically maintained in a predetermined spaced relationship with such surface, whereby the element may be 'maintained out of contact with the material so as not to be 4affected by the character of the material, `such as ccrrosiveness or viscosity, nor by waves in the material.

The invention has for another object theprovision of a level-sensitive element with electronic means, which maintain a predetermined spaced relationship between the element and the surface of the material in a container and which functions with such a small electric current inside the container that it may -be safely used where the fluent material is iniiammable.

Another object of the invention is to provide an exceedingly sensitive apparatus for determining the liquid level in a container with veryclose accuracy, say within a few hundredths of one per cent.

The invention has for another object the provision in apparatus of the class described of a level-sensitive element, comprising a wire element which is electrically resonant at a predetermined frequency `and on which a `high frequency signal is impressed, said element being detuned from resonance, when thewire and surface of the material are 'brought closer together, and means for effecting relative movement between said wire and surface, controlled by variation of a signal derived from said wire.

The invention has for a further object the provisi-on of a non-hunting level-sensitive apparatus comprising a two-phase low-frequency alternating current electric motor for effecting relative movement between the level-sensitive resonant wire and the surface of the fluent material in the container, either by raising Aand lowering the wire itself or by raising and lowering the level of the fluent material, a 4high-frequency lowpower oscillator inductively coupled to the resonant wire and tuned to resonance therewith, when 4the vwire and fluent material are relatively widely separated, said oscillator being-modulated with alternating current of the same -low freduency as the -motor and producing a signal in the resonant wire, and means for deriving from said signal a low frequency signal, which is amplified and transmitted to the motor to oppose the alternating currentsupply to various degr-ees, according to the spacing of said wire and level, and cause the motor to turn in one direction or the other as necessary to maintain the wire and level in said predetermined relationship and `to stop when such relationship is attained.

Theseand other objects will more particularly appear from the following description of illustrative examples of the invention in the accompanying drawings, in which:

Figs. l, 2,3 and 4 are small-scale, fragmentary sectional elevational Views illustrating various ways cf mounting the level-sensitive element of the apparatus in a container of fluent material;

Figs. 5 and 6 are sectional views taken on the lines 5 5 and .6 6 of Fig. l;

Fig. v7 is aperspective View, with parts broken away, showing the level-sensitive element and its mounting in its shield together with the coupling loop and detector, which are mounted in the shield;

Fig. 8 is a diagrammatical View of one form of apparatus used to operate a control switch;

Fig. 9 is a diagrammatical View of another form of apparatus used to operate a reversible alternating current motor; and

Fig. 10 is a perspective view, with parts broken away, of another vform of level-sensitive element.

Referring to these drawings and first to Figs. l to 4 thereof, the invention makes use of a levelsensitive element l, which is suitably mounted ina container 2 of any desired kind of fluent material '3, liquid or solid. The sensitive portion A. of this elementis mounted generally parallel with and spaced above the surface 5 of the material in the container. The spaced relationship of the portion 4 and surface 5 is maintained by suitable meansactuated by the signal derived, as will be later described in detail, from the sensitive element I. Such means may, for example, consist of an electric motor 6 (Fig. 5) driving, preferably throughsuitable gear reduction mechanism indicated ,at '7,a sprocket orpinwheel 3, carrying a chain or tape 9 (Fig. l), from one end of which the .sensitive element l and its enclosing shield i0 are suspended. The shield is in the form of anvinverted can with the end adjacent the surface of the fluent materialnormally open although it could, if desired, be closed by a thin membrane of insulating material. The other end of tape 9 carries a counterweight Il. Theshaft 8' of sprocket 8 drives by bevel gearing i2 a vertical shaft I3, which in turn drives by bevel gearing I4 a shaft I5, which operates a suitable indicator I 5, as for example one of the number wheel type, showing the level of material in the tank. The motor 5 will turn in one direction or the other to raise or lower the sensitive element I as may be required to maintain the predetermined spaced relationship. Alternatively, the motor 6 may, as shown in Fig. 2, drive a pump Il which is connected by a pipe I8 with a supply of the material and by a pipe I9 with the lower part of a tank in which the element I is stationarily mounted. The pump can force material into or draw material out of the tank to raise or lower the level 5 of the material in the tank as required to maintain the desired predetermined spacing between the surface of the material and the stationary sensitive element. The signal derived from the latter may also be made, as shown in Fig. 3 and as will later be described in detail, to start and stop a non-reversing motor 2 I, which operates a pump 22 to draw in material from a suitable supply through a pipe 23 and force it through a pipe 24 into a tank 25 in which the sensitive element I is stationarily supported. As the material in the tank is used up and its level falls, the motor 2| will operate the pump to bring the level back to the desired point. The sensitive element is maintained out of contact with the fluent material in the forms shown in Figs. 1, 2 and 3, which is very desirable when the material is dirty, corrosive or very viscous. However, the sensitive element may in some cases be submerged, as shown for example in Fig. 4, Where the sensitive element is arranged to respond to the level 25 of water 21 contained in a container 23 below a body of oil 29. Here the element I will be immersed in the oil but its sensitive portion 4 is stillmaintained in a predetermined spaced relationship above the level 26 of the water to which it is tuned to respond, as will later appear. With the Fig. 4 arrangement, the motor 6 may be arranged to raise and lower the sensitive element and indicate the level of the water or the sensitive element may be fixed and the motor may operate a pump to maintain the desired levelof water in either the manner of Fig. 2 or Fig. 3. In Figs. 1 and 4, the container, in which the sensitive-element is mounted, is a vertical tube, connected like a gauge glass, at top and bottom'to a larger tank 30, while in Figs. 2 and 3, the element I is mounted directly in the tank itself.

rIhe sensitive element itself is best shown in Fig. '1, which is a perspective view taken as if looking into the open end of a can-like shield i3, while the same is inverted from its normal working position. The sensitive element consists of a wire element which is electrically resonant at some predetermined high frequency, as for example, 200 megacycles. The wire is bent into the form shown and supported on insulators 3 I, each xed at one end to a post 32 the other end of which is iixed to the head of the shield I0. These insulators have metallic inserts 33 to which the wire is soldered. The sensitive portions 4 of the resonant wire consist of two portions each having an arc and a chord-like part which together mark oi an almost complete segment of a circle. All these parts lie in one horizontal plane, which is parallel with and spaced inwardly a short distance, say a sixteenth of an inch, from the plane of the open end of the shield. The ar-c-like parts lie in spaced concentrical relation with the peripheral wall of the shield. The

4 chord-like parts are parallel. The junction be` tween each chord-like and arc-like part is soldered to an insert 33 of an insulator 3|. From the other end of each chord-like part the wire is bent vertically inward, as at 34, and then horizontally, as at 35, in underlying relation with the chord-like parts. The two horizontal parts 35 are joined by a coupling loop 36, the mid-point of which is connected by a wire 3l to the head of the shield I and thus grounded. The junction of each part 34 with a part 35 is soldered to insert 33 of an insulator 3|. Within the shield II) is a wire loop 38 in xed coupling relation with loop 33 and the ends of this loop are connected by wires 39, which connect with the terminal ends of the wires 40 of a two-conductor, radio-frequency cable. This cable has a shield 4I, which is shown later in the diagrams and which is suitably connected to shield I0. Also contained within the shield I0 is a rectifier or detector 42 which is of the crystal diode type, suchv as a sealed-in-glass Germanium rectier which is desirable because it is small, requires no power supply and is suitable because of its low capacity for operation at the very high irequencies used. One terminal of this rectier is connected by a wire 43 to the mid-point of coupling loop 38 and the other terminal is connected by a wire 44 to the resonant wire I at a suitable point and as shown to one of the horizontal sections 35.- When a signal is impressed on the resonant wire, by means to be later described, the voltage is greatest at the free ends 45 of the wire and progressively decreases to zero at the mid point of the wire. The tap 44 is selected to transmit to the rectifier 42 the desired'fraction of the signal voltage, which may for example be about of the full voltage. Y

Fig. 8 shows a form of the apparatus, which can be used to actuate a switch such'as 46, which in turn may control a motor such as the above described pump-actuating motor 2 l ,shown herein as supplied from a volt, 60 cycle alternating current source. Ahigh frequency signal is impressed on the resonant wire I by any suitable means, such as the oscillator 41, which is of well known form and the parts ofwhich are entirely enclosedwithin a Asuitable shield, indicated by the dotted lines 48. This is a low-power oscillator having a frequency range sufficientA to include the natural frequency at which wire I is resonant. The plate circuit coil of the oscillator consists of a piece of copper tubing 49, the semicircular end of which forms a coupling loop 50. This loop is loosely coupled by an adjustable loop 5I to the transmission wires 4G, above described, and thus to loop 38 which is coupled as heretofore described, to the loop 38 of the resonant wire I. The tank circuit of the oscillator includes a variable condenser 52 by means of which the oscillator can be tuned to resonance with the sensitive element I. Power for the oscillator -is derived from a full wave rectiiier 53, supplied from the secondary 54 of a transformer 55, the primary 56 of which is connected as shown to the above mentioned alternating current source. The mid point of secondary 54 is grounded and its ends are connected one to each of the plates 51 of the 'rectiiiein The transformer also has a secondary 58 to supply current at 5 volts to the cathode 53 of the rectifier and a secondary 60 to supply current at 6I volts to the heating filament 6I of the oscillator tube 52. The rectifier 53 supplies direct current at 200 volts to the oscillator through a connection 63, which includes a small radio frequency choke 64 and is connected to one side of a by-passcone. denser 65, the other side of which is grounded;

the latter. is relatively remotefromthgsurface of thefluent materialin the container. In the present example, the effect of the iiuent material on' the .resonant wire would be negligible kat a distance of saysix inches,-therewould'be a slight effect-at a distance of three inches and a rapidly increasing effect. as theY ldistance decreases... A minute amount of power, sayfor example,- of the orderofl/mo ofa watt, is transferred to the wire l when the latterv is in resonance andata small voltage, say in the neighborhood of one volt. The presenceof the fluentmaterial detunes the .wire because its dielectric constantis higher than air. A portion of the signal passes through the rectilier 42, through the transmission wires lieto coupling loop `5l and by a wire 58 to oneY terminal of the coil EBS of. a very sensitive relay which actuates switch llt, closing it when the space between the wire lr andsurface 5 of the uent material eX- ceeds the. predetermined spacing., which-mayin this example be one inch, and increases the signal suiiciently for the purpose. The wire 53 has a small radio-frequencyv choke le therein and is connected to one side of a by-pass condenser 1i the other side of which is grounded. Thischoke 1B and condenser?! are .both located within the shield 4S, and arefor the purposeof excluding the high frequency ourrentfrom .the wire d8. For insurance a condenser 12 is connected between the wire 63 at a point adjacent relay coil til-and the ground for the same purpose. The wire et has a shield 13,. which is connected to the shield gi8 of the oscillator. It will be clear that, as the spacing between the sensitive portions l of the resonant wire l and the surface 5 of the iiuent material increases, the amplitude of the signal in wire l increases until the portion of the signal derived from wire l and transmitted to the very sensitive relay 5e is sunicient to close. switch t@ and start the motor 2l. Obviously, the relay could be madeto open the switch, whenever such action is desiredk for a particular control.

Fig. 9 shows another form of the invention which affords an exceedingly sensitive antihunting control for a reversible motor, such as the above mentioned motor t. A high frequency signal is produced by the same oscillator 41, but the signal is modulated with 60 cycle alternating current. The coupling between the resonant wire l and the oscillator is the same as described in connection with Fig. 8; the saine kind of detector is used and the signal derived is transmitted in the same mann-er through theshielded wire 6BY to a suitable amplifying means 88, to be later described, which amplies the low-frequency 60 cycle signal sufficiently to control the movement of the alternating current motor As before, power Vfor the oscillator is derived from a fullwave rectifier i5, which is supplied from the secondary 'it of a transformer r11, theprimary 13 of which lis connected to a 115 volt 60 cycle alternating current source. The mid point of the secondaryit is grounded `and itsl endsare `connected one to each of theplates 19 of the rectifier. This ,rectifier supplies the 300 volt direct .current needed for the amplifying means to be described and this voltage is reduced to the 200 volts, needed for the oscillator, by a resistor 89 included in series in thgwire 8l, whichconnects the cathode.

820i the rectier'to the plate loop49 of the osci1- later.. Also, included in series in this Wire 8| is the secondary 83 of a transformer 84, the primary 85 of which is connected-to the above mentioned alternating current source. This secondary supplies 60 cycle current at '10 volts to effect a fifty per cent modulation of the high frequency signal produced by the oscillator. rihe transformer 11, as before, has other secondaries B and 81 to respectively supply current at i'ive voltsl tov the cathode of the rectiler and current at 6 volts to the heating filament of the oscillator and the heating filaments of the amplifier tubes to be described.

The rectifier d2 derives a portion of the signal from the resonant wire l and transmits it asbefore to wire E8, the high frequency components of the signal being excluded, as before, by a choke FQ and a by-pass condenser 1l. The wire, in this case is connected to an amplifying means representedas the elements within the dotted line enclosure 53. Wire 68 is connected to one 'terminal of the input control resistor 8&3 of an ampliier tube The other terminal of the resistor is grounded and a by-pass condensor Si! is connected between the input connection to the grid di of tube 3S and the ground. The amplifying means is of a conventional form, designed for amplification of 60 cycle current with maximum efficiency. Only the alternating current component of the detected signal is amplified. The ainplifying means comprises two triode stages, using the tube and a tube S2, an interstage transformer a power amplifier tube 9,4 and an out put transformer S5 and is supplied with 300 volt direct plate current from the full-wave rectifier A gain of about 2000 is effected, providing across the secondary Se of transformer S3 a 60 cycle signal of approximately 32 volts when the wire is in resonance. VA transformer 91, the primary tof which is connected to the 115 volt 60 cycle alternating current supply, has its secondary e9 connected to a potentiometer led, from which a 60 cycle voltage is derived and applied in series with the secondary inthe grid Vcircuit of the power ampliiier tube. The transformer 84, which suppliesthg 60 cycle alterna-tingl current modulation for the oscillatorand transformer elv which supplies adjustable cycle alternating current bias for the power amplifier' grid are so connected thatsignal acrossV the secondary Se is opposite in polarityto the alternating current derived from secondary 99. A steady bias ofapproxinately 18 volts is simplied in the conventional mannerby the-coil lei of a relay` Iili; The signal across secondary for example, vary from a mum oi about 32 volts, when the vsensitive element i is in resonance, to zero; The derived from secondary imay, for exampic, be constantat say 20 volts. The differential voit is amplified sayZ times ,in tube Se and ,reduced one-half in transformer d, appearing in lthe secc-nondary HB2 as at least 120 volts, whenthe wire is in resonance. The secondary |52 is connected to onewindingtilt of the two-phase motor1 5, and has bridged across it aA power factor correc-V tion condenser l l5. The other windingv led of the motor is connected to the voit, cycle alternating current supply through a condenseriii. When` the wire i is in resonance, the 60 cycle current applied to winding l will cause-themotor 6 to turn in thedirection necessary to .lower the wire l. surface .of thefluent material, the'wire. is detuned As the wire i is lowered towardthe l to various degrees and the amplitude of the signal, derived and amplified as described, will decrease and eventually the signal voltage across secondary 96 will equal the voltage derived from transformer secondary 99 and the differential voltage becomes zero, when the motor 6 will stop with the wire located at say one inch above the level of the fluent material. Now, if the level rises, the voltage across secondary 96 will become less than that derived from secondary 99 and the differential voltage applied to winding |03 will be in an opposite direction, whereby the motor will reverse and lift the wire I until a balance is again obtained.

The motor 6 may, of course, as above described, operate to raise and lower the level of the fluent material instead of lowering and raising the Wire I.

The relay coil serves two purposes. It acts as a cathode bias resistor for the power amplifier tube 90 and also actuates a switch |06 controlling two indicator circuits connected to the 115 volt alternating current source. When the apparatus is in operation, coil iI will be energized and cause switch |00 to engage a contact |01 and close the circuit to an indicator lamp |00, which indicates that the apparatus is working properly. Deenergization 0f coil |0I will cause switch |06 to leave contact |01 and open the circuit to lamp |98 and to engage a contact |09 and close a circuit to a lamp IIO, which indicates that the apparatus is not in operation. On failure of the power supply, burn-out of rectifier tube 15 or transformer 16, or short circuit of the filter condensers I I I or I l2, or failure of the power amplifier tube, the relay IIlI will extinguish the operating indicator |08 and illuminate the danger indicator II.

The transformer 55, oscillator 41. full-wave rectifier 53 and relay 69 of the Fig. 8 apparatus or the oscillator 41, transformers 11, 84 and 91, full-wave rectifier 15, amplifying means 88, and relay |06 of the Fig. 9 apparatus may be housed, as shown in Fig. l, in a box III into which lead the alternating current supply wires I I2' and from which lead wires I I3 to connect with motor 6. The shielded radio-frequency cable 4I, indicated in Figs. 1, 5 and 6 by a single line, extends from the inverted can-like shield I0 to the housing II I and may be mounted in any suitable way to pay out the cable as the shield and the sensitive element housed therein descend in the container and to take up the slack as such shield and element rise. An illustrative example of one mounting, suitable for the purpose, is shown in Figs. 1 and 6. The cable extends upwardly from shield I0 in the container 2 and part way around a pulley I I4, fixed to the shaft 8.4 From the pulley I I4, the cable passes part way around the first of a series of seven sheaves I I6, all loosely mounted on an arbor II1, suitably supported from a housing IIS. The cable then passes downwardly around the lower part of the first of a series of seven sheaves I I9, all loosely mounted on an arbor |20, which is pulled downwardly by a Weight I2 I The cable then passes upwardly and over the second sheave IIB, then downwardly around the second sheave I I9 and so forth until it finally leaves the seventh lower sheave I I9 and passes horizontally across to a pulley |22 and then downwardly to the housing III. It will be clear that, as the sensitive element I and its shield I0 move downwardlythe arbor |20 will rise toward arbor II1 and provide the cable necessary and that when this element and its shield rise, the weight I2I will draw arbor |20 downwardly to take up slack in the cable.

In Fig. 10, a modified form of level-sensitive element is shown. This view is taken similarly to Fig. '1 and shows the parts inverted. Here,-a resonant wire I has a sensitive portion 4' of nearly circular form mounted in spaced coaxial vrelation with the peripheral wall of shield I0 and in a plane close to and parallel with that in which the open end of the shield is located. This portion 4' is supported by soldering it to metallic inserts 33 in insulators 3| mounted on posts 32 secured to the head of shield I0'. As before, the wire I has a coupling loop 36', one end of which is grounded by a wire 31 to the head of the shield. A coupling loop 38', related to loop 3B', is connected by wires 39' to the wires 40 of the radio frequency cable. The crystal diode detector 42 is connected by wires 43' and 44 to the mid point of coupling loop 38 and to the resonant Wire I at a point suitably spaced from the ground connection. This arrangement is a quarter Wave resonant wire instead of the half wave resonant wire shown in Fig. 7. It uses only half the wire needed with the Fig. 7 form and eliminates the half that did not have the tapped connection to the rectifier. This resonant wire is easier to tune because it does not have two sections as does the half wave resonant Wire of Fig. '7, which sections must be exactly matched in order to be tuned to resonance.

The invention provides an extremely sensitive control which responds to slight variations in the level of the material in a container and will determine the level with very close accuracy. Since the sensitive element follows the fluent material up and down in spaced relation with the surface of the fluent material, the control may be used with materials which would destroy the sensitive element on contact, such as acids or molten material. And close accuracy can be had in the measurement of liquids, which are very viscous or which by their nature would impair the movement of a level-sensitive element, requiring contact with the liuent material. It is useful for level determination in large tanks, where waves may be produced in the fiuent material that would affect the movement of a level-sensitive element in contact with the fluent material. Because of the extremely low voltage of the signal impressed on the resonant wire, the control can be safely used in containers of inflammable liquids. The control is also useful with such solid materials as will ow. The control as used for level determination with the reversible motor for moving the sensitive element, provides for movement of the element with any very slight change in level of the material and not in steps but with the uent material. The sensitive element follows the fluent material, as it moves up and down, and is virtually synchronized with it. The motor does not go on and on periodically. Rather its torque gradually increases from zero, when the level of the uent material is stationary, in one direction or the other accordingly as the level of the uent material rises or falls and immediately follows the level. Thus, close accuracy in measurement may be had. One of the conditions the control had to fulfill was to show the level in a deep tank with a variation of no more than one eighth of an inch in fifty feet, or approximately .02%.

As used for maintaining the level in a container constant, the invention is useful, as before, with inflammable liquids or iiuent material, whichn Woulddestroy the sensitive element if brought in Contact with it. The control is sensitive to slight variations in level and can provide closer regulation of the level Vin a tank since the sensitive element does not have'to be moved b'y the fluent material and remains out of contact with it.

The sensitive element may be tuned to respond to lthe level of one of a plurality'of liquids in a tank, providing the liquids have different leleotricalV properties. Thus, in thev case ofthe apparatus shown in Fig. fl, oil is an insulator while Water is a conductor electricity. Therefore, the water will detune the resonant wire. Thus, vthe Sensitive element caribe made to respond to variations in the level 0f the water, unaifected-by the presence .of the oil above it.

The 'invention thusprovides a level-sensitive apparatus for the control of the level of fluent material in a container, or for the determination '0f such level, 'and this control is characterized by the use of a resonant wire; on which a high frequency signal is impressed and which is tuned to resonance while substantially spaced from the fluent materialin a 'container 'and therefore, becomes detuned, when in close proximity to the surface of the material, the detuning providing a change rin the amplitude of the signal which change is utilized to actuate a suitable control to move the level of the fluent material or to move the -sensitive element toward or away from the fluent material, as required.

What is claimed is:

1. .The combination with a container of fluent material, of a wire velement electrically resonant at a predetermined :frequency and mounted in said container` spaced above the surface of said material a predetermined distance, vmeans for effecting relative movement between said .element and surface, said means including a twophase low-frequency electric motor having two windings, separate circuits for supplying lowfrequency alternating current in proper vphase relation to the windingsof said motor, a high-frefluency low-power oscillator inductivelycoupled to said element and tuned to resonance therewith when the element is relatively remote from the surface of said material and detuned t0 various degrees las the element approaches said surface, means for modulating the high frequency oscillations with alternating current of the low freduency used forsa-id motor, a rectifier receiving part of the signal from the resonant element, and means for .amplifying the rectified signal Vand transmitting it to'one of said circuits in a diretionto oppose the first-named alternating current supply thereto, the full amplified signal when said oscillator-and element are in resonance being greater than the first-named alternating current supply to the last-named circuit and causing the motor to effect relative Vmovement in the first-named direction until the amplified signal and iirst-named alternating current `balance, when 'said element is spaced from said surface 'by said predetermined distance, further reln ative movement in the first-named direction decreasing the amplined signal to less than the firstnarned alternating `current supply and causing the motor to effect relative movement in the opposite direction until said-.element and surface are separated by said predetermined distance.

2. The combination with a container of fluent material,l of a wire element electrically resonant ata predetermined frequency and ylocated in said container'spaced above thesurface of said material -a predetermined distance, means for moving said element up and down in the container to follow variations in the level of said material, said means including a two-phase low-frequency electric motor having two windings, separate circuits for supplying low-frequency alternating current in proper phase relation to the windings of said motor, a high-frequency low-power oscillator inductively coupled to said element and tuned to resonance therewith when the element is relatively remote from the surface of said material and detuned to various degrees as the element approaches said surface, means for modulating the high frequency oscillations with alternating current of the low frequency used for said motor, -a rectifier receiving part of thesignal from the resonant element, and means for amplifying said-part of the signal and transmitting it to one of said circuits in a direction to oppose the rst-named alternating current supply thereto, the full amplified signal when said oscillator and element are in resonance being greater than the first-named alternating current supply to the last-named circuit and causing the motor to lower said element until the ampliiied signal and first-named alternating current balance, when said element is spaced from said surf ce by said predetermined distance, further lowering of lsaid element decreasing the amplified signal to less than the rst-named alternating current supply and causing the motor to raise said element until it is spaced from said surface by said predetermined distance.

3. The combination with a container of fluent material, of a wire element electrically resonant at a predetermined frequency and having sensitive end portions and intermediate such end portions a coupling loop, a membersupporting said element in said container with the sensitive end portions spaced a predetermined distance above the surface of the fluent material, means for effecting relative movement between said supporting member and surface to maintain the predetermined spacing of said end portions and surface, a high-frequency oscillator with a coupling loop, a radio-frequency cable having a coupling loop on one end related to the oscillator loop and a coupling loop on the other end mounted on said supporting member and related to the first-named loop, whereby a high frequency signal from the oscillator may be impressed on said element, said element being tuned to resonance with said 'oscillator when said portions and surface are in another and wider predetermined spaced relationship, rthe relative movement of said portions and surface toward said first-named predetermined spacing detuning said element and decreasing the amplitude of said signal, a detector of the crystal Vdiode type connected to said element at a pointspaced from the mid point thereof and connected to the coupling loop of said cable on said supporting member', and a conductor connecting the other loop on said cable to said first-named meansand transmitting the detected signal to the firstnamed means for actuating the same.

Li, The combination with a container `of fluent material, of a wirek element insaid container electrically resonant at a predetermined frequency and spaced above the surface of said material a predetermined distance, said element with some of said material andthe gap between the element and material forming a first circuit, a high-frequency low-power oscillator, a second circuit including said oscillator and coupled to the lire-t circuit for producing in the latter a high-frequency low-voltage signal, means for effecting relative movement between said element and surface, the first circuit being tuned to resonance with the second circuit when said element and surface are in another and wider predetermined spaced relationship, the relative movement of said element and surface toward the first-named predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, a detector, an electrical operator, and a third circuit including said detector and operator and coupled to the first circuit through said detector to receive a portion of the signal therefrom and transmit it to the operator, whereby the latter may be actuated by a variation in the amplitude of the detected signal.

5. The combination with a container of fluent material, of a wire element in said container electrically resonant at a predetermined frequency and spaced above the surface of said material a predetermined distance, said element with some of said material and the gap between the element and material forming a rst circuit, a high-frequency low-power oscillator, a second circuit including said oscillator and coupled to the first circuit for producing in the latter a highfrequency low-voltage signal, means for effecting relative movement between said element and surface, means for tuning said oscillator and its circuit to resonance with said element and its circuit when said element is relatively remote from the surface of said material, the relative movement of said element and surface toward the first-named predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, a rectifier connected to said element, and a third circuit including said rectifier and firstnamed means and coupled through said rectifier to the rst circuit to receive a portion of the signal therefrom and transmit it to the firstnamed means, whereby the latter may be actuated by a variation in the amplitude of the detected signal.

6. The combination with a container cf fluent material, of a wire element in said container electrically resonant at a predetermined frequency and spaced above the surface of said material a predetermined distance, said element with some of said material and the gap between the element and material forming a first circuit, a modulated high-frequency low-power oscillator, a second circuit including said oscillator and coupled to the first circuit for producing in the latter a high-frequency lowvoltage signal, means for effecting relative movement between said element and surface to maintain the predetermined spacing, the first circuit being tuned to resonance with the second circuit when said element and surface are in another and wider predetermined spaced relationship, the relative movement of said element and surface toward the first-named predetermined spacing detuning the rst circuit and decreasing the amplitude of the signal, a detector, a third circuit including said detector and coupled to the first circuit through said detector to receive a portion of the signal therefrom, and means for amplifying the detected signal and transmitting it to the first-named means to control the actuation of the same.

'7. The combination with a container of fiuent material, of a wire element in said container electrically resonant at a predetermined frequency and spaced above the surface of said material aV predetermined distance, said element with some pled to the first circuit for producing in the latter a high-frequency low-voltage signal, means for raising and lowering said element to maintain the predetermined spacing, the first circuit being tuned to resonance with the second circuit when sai-d element is raised above said surface in another and wider predetermined spaced relationship, the lowering of said element toward the first-named predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, Ia detector, a third circuit including said detector and coupled to the first circuit whereby to receive a portion of the signal from the first circuit, and means for amplifying the signal received by the third circuit and transmitting it to said first-named means to control the actuation of the same.

8. The combination with a container of fluent material, of a wire element electrically resonant at -a predetermined frequency, said 'element mounted in'fixed position in said container and spaced a predetermined distance Iabove the surface of said material, means for raising and lowering the level of said material in the container to maintain the predetermined spacing, said element with some of said material and the gap between the element and material forming a first circuit, la modulated high-frequency low-power oscillator, a second circuit including said oscill-ator and coupled to the first circuit for producing in the latter a high-frequency low-voltage signal, the first circuit being tuned to resonance with the second circuit when said element and surface are in another and wider predetermined spaced relationship, the relative movement of said element and surface toward the first-named predetermined spacing ydetuning the first circuit and decreasing the amplitude of the signal, a detector, a third circuit including said detector and coupled to the first circuit through the detector to receive a portion of the signal therefrom, and means for amplifying the `detected signal and transmitting it to the first-named means to control the actuation of the same.

9. The combination with a container of fluent material, of a wire element in said container electrically resonant at a predetermined frequency and spaced above the surface of said material a predetermined distance, said element with some of said material and the gap between the element and material forming a first circuit, a high-frequency low-power oscillator, Ia second circuitY including said oscillator and coupled to the rst circuit for producing in the latter a highfrequency low-voltage signal, means for effecting relative movement between said element and surface, means for tuning said oscillator and its circuit to resonance with said element and its circuit when said element is relatively remote from the surface of said material, the relative movement vof said element and surface tow-ard the rst-named predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, a ydetector of the crystal diode type connected to said element, and a third circuit including said detector and coupled by the latter to the first circuit to receive a portion of the signal therefrom and transmit it to the first-named means, whereby the latter may be actuated by a variation in the amplitude of the detected signal.

10. The combination with a container of fluent material, of a Wire element in said container electrically resonant at a predetermined frequency and spaced above the surface of said material a Ipredetermined distance, a metallic shielding caslng in which said element is ysupported and which encompasses said element except on the under side which faces said surface, said element with some of said material and the gap lbetween the element and material forming a first circuit, a high-frequency low-power oscillator, a second circuit including said oscillatoi` and coupled to the rst circuit for producing in the latter a high-frequency low-voltage signal, means for eifecting relative movement between said element and surface, means for tuning said oscillator and its circuit to resonance with said element and its circuit when said element is relatively reine-te from the surface of said material, the relative movement of said element and surface toward the first-named predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, a `detector of the crystal diode type connected to said element, and -a third circuit including said detector and coupled by the latter to the first circuit to receive a portion of the signal therefrom and transmit it to the firstnamed means, whereby the latter may be actuated by a variation in the amplitude of the dctected signal.

1l. The combination with a container of two immiscible liquids located one above the other, the upper and lower liquids being respectively electrically insulatingr and electrica conducting, of a wire element electrically resonant at a predetermined frequency, said element being 1" mersed in the insulating liquid and spaced a determined distance above the surface of the conducting liquid, said element with some of the conducting liquid and the gap between the latter and said element forming a first circuit, a high-frequency l-ow-power oscillator, a second circuit in-I cluding said oscillator and coupled to the circuit for producing in the latter a high-frequency low-voltage signal, means for effecting relative movement between said element surface, means for tuning said oscillator and its circuit to resonance with said element and its circuit when said element is relatively remote from the surface of said material, the relative movement of said element and surface toward the firstnamed predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, a rectier connected to said element, and a third circuit including said rectier and iirstnamed means and coupled through said rectifier to the rst circuit to receive a portion of the signal therefrom and transmit it to the iirstnamed means, whereby the latter may be actuated by a variation in the amplitude of the detected signal.

12. The combination with a container of two immiscible fluent materials located one above the other and having different dielectric constants, of a wire element electrically resonant at a predetermined frequency, said element being immersed in the upper material and spaced a predetermined distance above the surface or" the lower material, said element with some of the lower material and the gap between the latter and said element forming a rst circuit, a high-frequency low-power oscillator, a second circuit in- 0 Number 14 cluding said oscillator and coupled to the rst circuit for producing in the latter a high-ireduency .row-voltage signal, means for effecting relative movement between said element and surface, means for tuning said oscillator and its circuit to resonance with said element and its circuit when said element is relatively remote from the surface of said material, the relative movement of said element and surface toward the rstnamed predetermined spacing detuning the first circuit and decreasing the amplitude of the signal, a rectiiier connected to said element, and a third circuit inclucing said rectifier and firstineans and coupled through said rectifier to the nrst circuit to receive a portion of the signal therefrom and transmit it to the first-named means, whereby the latter may be actuated by a variation in the amplitude of the detected sigconibination with a container of fluent material, of a wire element of the quarter-wave type electrically resonant at a predetermined frequency, said element having a grounded end and a end together with a coupling loop adjacent the grounded end, a member supporting said element in said container with a portion of it between said loop and free end spaced a predetermined distance above the surface of the fluent material, means for effecting relative movement between said supporting member and surface to maintain the predetermined spacing of said element and surface, a high-frequency oscillator with a coupling loop, a radio-frequency cable having a coupling loop on one end related to the oscillator loop and a, coupling loop on the other end mounted on said supporting member related to the Erst-named loop, whereby a highfrequency signal from the oscillator may be impressed on said element, the latter being tuned to resonance with said oscillator when said portion and surface are in another and wider predetermined spaced relationship, the relative movement of said portion and surface toward said firstnamed predetermined spacing detuning said element and decreasing the amplitude of said signal, a detector of the crystal diode type connected to said wire at a point spaced from the mid point thereof and connected to the coupling loop of said cable on said support, and a conductor connecting the other loop on said cable to said first-named means and transmitting the detected signal to the first-named means for actuating the same.

HARRY TAP?. THEODORE J. MESZ-l.

CES DETER The following references are of record in nie of this patent:

the

UNITED STATES PATENTS Number FOREIGN PATENTS Country Date 750,075 France Aug. 3, 1933 

